Esterified alkoxylated glycerin and other esterified alkoxylated polyols have recently been identified as useful reduced calorie fat substitutes. Compounds of this type, which are described more fully in U.S. Pat. No. 4,861,613, are substantially resistant to hydrolysis upon digestion owing to the high proportion of linkages in which the carbons adjacent to oxygen in the fatty acid ester groups are secondary or tertiary in structure. In a preferred embodiment of such substances, the structure may be represented as follows: ##STR1## wherein G is a glyceryl radical, n is from about 1 to 3 on average, and R is a long chain paraffinic or olefinic hydrocarbon radical derived from a fatty acid.
However, the ability to use esterified alkoxylated polyols of this type at relatively high concentrations in food compositions is somewhat limited by the pronounced resistance of such substances to digestion. Since the esterified alkoxylated polyols are hydrolyzed and absorbed to only a very limited degree, they tend to retain their oil-like physical characteristics after ingestion. Consumption of large amounts of the fat substitutes can result in short bowel transit times and undesired laxative effects.
To enhance the acceptability of fat substitutes of this type, a modified esterified alkoxylated glycerin has been developed which is somewhat less resistant to enzymatic hydrolysis than previously known esterified alkoxylated glycerins and yet still has significantly reduced calorie availability as compared to a conventional fully digestible triglyceride lipid. This modified esterified alkoxylated glycerin has one fatty acid ester group attached directly to the end carbon of the glyceryl radical. Since this ester group is derived from a primary hydroxyl group, it is readily hydrolyzed upon ingestion, rendering the compound less fat-like in character owing to the loss of a long-chain fatty acid group.
The other two ester groups in the esterified alkoxylated glycerin are attached to the glyceryl radical through polyoxypropylene segments and thus are resistant towards enzymatic hydrolysis. The structure of a preferred embodiment of the esterified alkoxylated polyol is as follows: ##STR2## wherein R is a long-chain hydrocarbon radical derived from fatty acid.
The synthesis of such esterified alkoxylated monoglycerides is not straightforward. Esterified propoxylated glycerin may be prepared by reacting glycerin with propylene oxide in the presence of a basic alkali metal catalyst to form a propoxylated glycerin. The propoxylated glycerin is then esterified with a fatty acid compound such as a free fatty acid, fatty acid ester, or fatty acid halide. Using this synthetic approach, however, it is not possible to have an ester group attached directly to the glyceryl residue since the propylene oxide tends to add in a random fashion to all three hydroxyl groups of the starting glycerin: ##STR3##
A possible alternative method of preparation of an esterified alkoxylated monoglyceride would be to propoxylate a fatty acid monoglyceride and then esterify the secondary hydroxyl groups of the propoxylated monoglyceride: ##STR4## However, when this procedure is attempted, the product obtained is similar to the esterified propoxylated glycerin known in the prior art wherein oxypropylene units are present between the glyceryl radical and all three of the ester groups. Apparently, transesterification readily takes place under the reaction conditions necessary to achieve propoxylation of the fatty acid monoglyceride: ##STR5##
Thus, it is apparent there is a great need for processes whereby an esterified alkoxylated polyol having at least one ester group attached directly to the polyol residue may be readily prepared.